Magento – Unable to submit your request Magento Contact Form Error

I keep getting this error when trying to submit my contact form:
"Unable to submit your request. Please, try again later"

I'm using default Magento contact form.

Does anyone know where to start with this? It was working a few days ago and now it's not.

I've also been having issues with sales emails sending.

Thanks in advance.

In the example below I tried to go to an order and click "send email" then I tired to fill out the contact form on the front end…

# root path of bitnami installation replaced with [root]

2013-08-23T11:06:05+00:00 ERR (3): 
exception 'Zend_Mail_Transport_Exception' with message 'Unable to send mail. ' in [root]/lib/Zend/Mail/Transport/Sendmail.php:137
Stack trace:
#0 [root]/lib/Zend/Mail/Transport/Abstract.php(348): Zend_Mail_Transport_Sendmail->_sendMail()
#1 [root]/lib/Zend/Mail.php(1194): Zend_Mail_Transport_Abstract->send(Object(Zend_Mail))
#2 [root]/app/code/core/Mage/Core/Model/Email/Template.php(454): Zend_Mail->send()
#3 [root]/app/code/core/Mage/Core/Model/Email/Template.php(506): Mage_Core_Model_Email_Template->send(Array, Array, Array)
#4 [root]/app/code/core/Mage/Core/Model/Email/Template/Mailer.php(79): Mage_Core_Model_Email_Template->sendTransactional('sales_email_ord...', 'sales', Array, Array, Array, '1')
#5 [root]/app/code/core/Mage/Sales/Model/Order.php(1321): Mage_Core_Model_Email_Template_Mailer->send()
#6 [root]/app/code/core/Mage/Adminhtml/controllers/Sales/OrderController.php(130): Mage_Sales_Model_Order->sendNewOrderEmail()
#7 [root]/app/code/core/Mage/Core/Controller/Varien/Action.php(419): Mage_Adminhtml_Sales_OrderController->emailAction()
#8 [root]/app/code/core/Mage/Core/Controller/Varien/Router/Standard.php(250): Mage_Core_Controller_Varien_Action->dispatch('email')
#9 [root]/app/code/core/Mage/Core/Controller/Varien/Front.php(176): Mage_Core_Controller_Varien_Router_Standard->match(Object(Mage_Core_Controller_Request_Http))
#10 [root]/app/code/core/Mage/Core/Model/App.php(354): Mage_Core_Controller_Varien_Front->dispatch()
#11 [root]/app/Mage.php(683): Mage_Core_Model_App->run(Array)
#12 [root]/index.php(87): Mage::run('', 'store')
#13 {main}
2013-08-23T11:07:48+00:00 ERR (3): 
exception 'Zend_Mail_Transport_Exception' with message 'Unable to send mail. ' in [root]/lib/Zend/Mail/Transport/Sendmail.php:137
Stack trace:
#0 [root]/lib/Zend/Mail/Transport/Abstract.php(348): Zend_Mail_Transport_Sendmail->_sendMail()
#1 [root]/lib/Zend/Mail.php(1194): Zend_Mail_Transport_Abstract->send(Object(Zend_Mail))
#2 [root]/app/code/core/Mage/Core/Model/Email/Template.php(454): Zend_Mail->send()
#3 [root]/app/code/core/Mage/Core/Model/Email/Template.php(506): Mage_Core_Model_Email_Template->send('christinarule@g...', NULL, Array)
#4 [root]/app/code/core/Mage/Contacts/controllers/IndexController.php(104): Mage_Core_Model_Email_Template->sendTransactional('contacts_email_...', 'custom2', 'example@g...', NULL, Array)
#5 [root]/app/code/core/Mage/Core/Controller/Varien/Action.php(419): Mage_Contacts_IndexController->postAction()
#6 [root]/app/code/core/Mage/Core/Controller/Varien/Router/Standard.php(250): Mage_Core_Controller_Varien_Action->dispatch('post')
#7 [root]/app/code/core/Mage/Core/Controller/Varien/Front.php(176): Mage_Core_Controller_Varien_Router_Standard->match(Object(Mage_Core_Controller_Request_Http))
#8 [root]/app/code/core/Mage/Core/Model/App.php(354): Mage_Core_Controller_Varien_Front->dispatch()
#9 [root]/app/Mage.php(683): Mage_Core_Model_App->run(Array)
#10 [root]/index.php(87): Mage::run('', 'store')
#11 {main}